Compact white light sources are extensively used for backlight modules of scanners or color panel displays. Therefore, the improvement of white light sources is also great issue for the related industries.
FIG. 1 shows an exploded view of an edge backlight module 10, which comprises a panel light guide 100, a lamp 102 on one side of the panel light guide 100, a lamp reflecting plate 104 on outer side of the lamp 102, and a reflecting plate 106 on bottom of the panel light guide 100. The edge backlight module 10 further comprises prism and light diffusing plates (not shown). The edge backlight module 10 can be used for a panel display and the lamp 102 can be a CCFL for emitting white light. The light emitted from the lamp 102 is reflected by the lamp reflecting plate 104 and then guided by the panel light guide 100, the prism and the light diffusing plate for forward emission. The forward emission can provide backlight for an LCD device.
However, the CCFL has the following problems: (1) complicated circuit design due to a high driving voltage; (2) unstable illumination that varies with the temperature; and (3) a dead zone present at both end of the lamp. When the edge backlight module 10 is used for a scanner, the scanning quality is degraded due to the suboptimal conditions mentioned above.
The rapid progress in LED technologies increases the possibility of replacing the CCFL with an LED, and the white LED can be provided by in the following two ways:
(1) When a blue LED is used with yellow phosphor, the illumination efficiency thereof is influenced by the phosphor.
(2) When red, green and blue LEDs are used together to mix a white light, the color temperature of this white LED may be unstable due to process variation of the LEDS.